Common electric motors and/or generators operate on electromagnetic principles, and use magnetic fields from permanent magnets and/or wound-wire electromagnets to generate current or mechanical motion. A simple example of this principle is shown in FIG. 1A, where a coil 100A carrying current I winds about a U-shaped “core” 102A made of iron (or another ferromagnetic material, i.e., one which is magnetizable), and which has an iron (or other ferromagnetic) post 104A situated within the core. The current converts the core 102A into an electromagnet with magnetic flux density B, pulling the post 104A rightwardly into the gap 106A with F reluctant force. Thus, electric power causes mechanical motion. Conversely, mechanical motion can be converted to electric power: if current I is not supplied (and if post 104A is a magnet), pulling the post 104A away from the electromagnet 102A generates current I at the terminals of the coil 100A (assuming the terminals are connected to form a closed circuit). Common motors and generators use this concept by (for example) providing a series of coils about a ferromagnetic rotor (with the coils having axes which intersect at the rotor), and energizing the coils in sequence to spin the rotor (or spinning the rotor to generate electricity from the coils). Machines of this nature can be referred to as electromagnetic motors and generators, or more simply as inductive machines (“induction” referring to the generation of a magnetic field upon application of current). Inductive machines can also take the form of devices other than motors and generators; for example, electrical signals (e.g., data and/or power) can be transferred between parts, such as opposing coils, using magnetic fields.
Less well known are electrostatic (or electrodynamic) motors and generators, which can also be referred to as capacitive machines. These are devices that produce motion from electrical input, produce electricity from motion input, and/or transfer electrical signals (e.g., data and/or power) between parts, by using capacitance (the ability of a system to store electric charge) rather than induction. Stated differently, charge is moved by electric fields in capacitive machines, whereas charge is moved by magnetic fields in inductive machines. In simpler terms, capacitive machines rely on the “static cling” force that holds clothing together when it's pulled from a clothes dryer, whereas inductive machines rely on the magnetic force that holds magnets to iron. FIG. 1B illustrates a capacitive version of the inductive machine of FIG. 1A, wherein two fixed parallel conducting plates 102B are separated by an electrically isolating gap 106B. The plates 102B are connected to a voltage source to create electric field E between them. Pulling a conductive plate 104B through the gap 106B with force F generates electricity because the electric field induces an electric charge on the surface of the movable plate 104B, as well as exerting a force to move the plate 104B rightward. Thus, electric power causes mechanical motion. As with an electromagnetic system, if an external force can overcome F, the mechanical motion will generate electric power. Similarly to the electromagnetic system of FIG. 1A, this translational motion may be transformed into rotational motion. For example, a number of sets of parallel plates (like the plates 102B) might be arrayed along a circular path, and might have a rotor mounted to rotate about a shaft, with the rotor having arms (like the plate 104B) situated to travel between the parallel plates as the rotor rotates. If the parallel plates are sequentially charged, the arms of the rotor can be pulled about the circular path, converting electric power to mechanical motion. This capacitive machine may also be “run in reverse,” with rotation of the rotor generating electrical output. Desired levels of output torque or electric power can be generated by stacking further plates in the circular parallel array, and by connecting further rotors to the shaft. Capacitive machines of this nature hypothetically have a number of advantages over inductive machines, including reducing or eliminating the cost, bulk, and weight of magnets, ferrous materials, and low resistance copper windings. Moreover, the reduction in bulk and weight should have the secondary effect of allowing higher-speed operation. Higher electrical efficiencies should also be possible because capacitive machinery operates best at higher voltages, which typically provides lower conduction losses.
Despite the foregoing, capacitive machines are presently rarely used. This is in large part owing to the relatively low capacitive coupling that can practically be obtained between the moving and stationary plates of practical capacitive machines, resulting in low power density. Ideally, plates should be spaced as closely as possible in parallel relationship for greater capacitance (and thus greater power density), but practical difficulties limit plate spacings and parallel alignments: it is difficult to attain close (ideally micron-level) spacings between plates having large area, which is also beneficial for power density, and small thickness, which is beneficial for weight and speed, particularly owing to plate flexure. External influences such as temperature, vibrations, shaft end play, orientation (gravity), etc. can readily result in plate-to-plate contact and damage to (or destruction of) a capacitive machine, particularly at high operating speeds. The problem grows more acute as additional parallel plates are added to attain preferred power densities.